Heart rate fluctuation or heart rate variability is utilized as a method for determining a drowsiness level of a subject. For example, the method based on the heart rate fluctuation is used to continuously obtain a fixed number or more of heartbeat interval values, to calculate spectral density by frequency converting the obtained data row, and to determine the drowsiness level of the subject. It is possible to calculate the heartbeat interval between R waves, which have the largest amplitude in each heartbeat. The heartbeat interval is able to be calculated using a time interval of two R waves which are, for example, adjacent heartbeats, and is referred to as an R-R interval (RRI).
It is suggested to use a drowsiness detection device based on the technique for monitoring a vehicle driver. However, when the drowsiness detection device obtains the heartbeat signal of the driver in order to generate heartbeat interval data, there is a case where noise due to influence of vehicle vibration and the like is generated to result in an electrocardiograph signal including noise. In a case where the drowsiness detection device obtains the heartbeat interval data including noise, reliability of fluctuation analysis, which is carried out to convert the heartbeat interval data to a drowsiness value, is reduced because a target fluctuation component is reduced. When detecting noise, the drowsiness detection device specifies a range of heartbeat interval data which includes the noise, and continues calculation of the drowsiness value while correcting the fluctuation component.
Japanese Laid-open Patent Publication Nos. 2006-81840, 7-124140, 2003-339651, and 2013-123524, and International Publication Pamphlet No. 2008/149559 are examples of the related art.